clicking/grunting noise after cold start, intermittent

Occasionally when I start up Prius in the morning or after work, while the brake i depressed, I am getting a clicking/grunting sound (sounds like a series of short, fast grunts, actually) from under the dash on the passenger's side. It also happens when I have been driving awhile, when stopped at a traffic light, again with the brake depressed. It will go weeks with no noise, then do it several times in a day. HELP! Dealer is clueless because we can't replicate it. I have had the Prius over a year, and it is not one of the usual pump noises that I am accustomed to. I am going on a trip soon, and would like to have this resolved. I purchases Prius in Mississippi, but recently moved to Northern Virginia. Any ideas, you gurus out there?
Thanks in advance!